In the November 1987, Guitar Player interview George Harrison states, 'I realized somewhere down the line, I was playing these Gretsch guitars through these Vox amps, and in retrospect they sounded so puny. It was before we had the unwound third string, that syndrome, and because it was always done in a rush and you didn't have a chance to do a second take, we just hadn't developed sounds on our side of the water'. First of all we realize George was a very modest person and he was a great guitarist! Secondly, he was a huge fan of the early American rockers like Buddy Holly and he could have been a bit frustrated as he wasn't getting that Fender Stratocaster tone you can only get through an old tweed amp. Of course we loved that Gretsch/Rickenbacker/Vox tone of the British Invasion. Back to strings, from what I've read The Beatles and all the Mersey groups had access to round-wound strings and I believe they would have preferred them if they had a choice. But working musicians wouldn't always have the opportunity(or cash) when a string broke to replace the whole set or even chose between round or flat. They would replace broken strings one at a time with whatever was available. In the book, 17 Watts by Mo Foster, Albert Lee explains: "About a year before(Cochran and Eddy toured England) word had got around the way US players were stringing their axes, Duane Eddy and Eddie Cochran had told of how players would buy a regular set of strings and an extra first or light banjo string, move them over one and throw away the bottom string. This opened up a whole new world to us". Eddie Cochran was due to appear on a major show at Liverpool Stadium on May 3, 1960 with Gene Vincent, but was killed in a car accident on April 17, 1960 in England. So as early as possibly '59 some British guitar players were aware of light gauge strings. I think The Beatles would have been 'in the know' when it came to light gauge strings(and round wound strings) as early as '60-61. But being working musicians they probably didn't have the cash or opportunity to use them too often until they became huge. I guess it's anybodies guess as to when, where or what guitars they were using round-wounds on!